Boat propeller



Dec. 21, 1965 w. c. THOMPSON BOAT PROPELLER Filed April 17, 1964 NdE INVENTOR WILLIAM C. THOMPSON ATTORNEY 3,224,509 Patented Dec. 21, 1965 3,224,509 BOAT PROPELLER William C. Thompson, Freeport, N.Y., assignor to Columbian Bronze Corporation, Freeport, N .Y., a corporation of New York Filed Apr. 17, 1964, Ser. No. 360,586 5 Claims. (Cl. 170-1605) The present invention relates generally to propellers particularly adapted to propel water craft and to operate immersed in water. More specifically, it relates to boat propellers formed from synthetic plastic materials.

In accordance with a trend toward the production of smaller boats that have opened a mass market for economically price vessels, the direction of production of propellers for operating these boats has been toward more economically price propellers, both as original parts and as replacement parts for propellers that have been damaged.

Propellers formed from bronze have been considered most suitable and still are the standard in propellers for other than outboard powered vessels. With propellers best adapted for propelling smaller craft, particularly outboard motor-driven craft, however, bronze propellers have proved to be too heavy and to result in high inertia loads and lower efiiciency. This is especially true in the case of outboard motors having a horsepower of or less. The use of bronze also leads to galvanic corrosion when used in salt water with aluminum engine parts, and is relatively expensive. Sand cast aluminum propellers are also relatively expensive and die cast aluminum propellers, while less expensive than bronze or sand cast aluminum propellers, tend to break when they strike a submerged object, resulting in loss of one or more blades of the propeller. Loss of blades can result in a propeller that will not function with enough proficiency to propel its boat and thus lead to a disabled craft.

One apparent solution to high cost and other problems in the manufacture and operation of propellers has been to utilize plastic resins as the materials from which the propellers, specifically the propeller blades, are formed. First, almost any type of plastic material is more economical than either bronze or sand cast aluminum. It is usually less expensive than die cast aluminum. Second, most plastics will, upon being forced into contact with a hard object, tend to tear or deform rather than fracture. While deformation or tearing certainly detract from the effectiveness of the propeller, they do not result in a propeller that is unable to power its craft to the nearest repair or replacement facility. Other advantages of plas tic as a material from which to form propellers are the complete freedom of plastic resins from corrosion in salt water, and their light weight in comparison with the heavier metals and alloys. For all these reasons, plastic boat propellers have appeared especially adapted for driving inboard and outboard powered boats, but particularly for use with small outboard motors; yet thus far they have been found generally unsuitable and have received no general acceptance from boating enthusiasts.

One major reason for the failure in acceptance of plastic propellers has been the fact that the propellers are relatively flexible in comparison with their metal counterparts. Although various attempts have been made to reinforce the propeller blades and thereby stiffen the blades, it is still a fact that plastic blades flex when subjected to the pressure of water during use at low, medium and high boat speeds. This flexure adversely affects propeller operation, for the form of the propeller is much different from its form prior to flexing and so the propeller is not able to deliver full power in its deflected form. Attempts to mitigate deflection by increasing the thickness of the propeller blades have proved unsuccessful, because the thicker blades are inefficient and the plastic then tends to fracture instead of flex upon contact with a submerged obstruction.

It is, therefore a primary object of the present invention to provide a propeller form from a plastic material that will have all of the advantages of a plastic propeller but will maintain its power through all of the speeds at which the motor may be operated.

According to my invention, changes in flexur'e of a plastic propeller with varying motor r.p.m.s are compensated for, not attempted to be prevented. Compensation takes two forms: (1) compensations for variations in the pitch of the blades under power, and (2) compensation for variations in the rake of the blades under power. Specifically, variations in pitch under power are corrected by altering the pitch of each blade when the blade is static. The pitch is changed gradually, being the greatest near the hub to which the blades are fixed and then diminishing to a minimum at the tips of the one or more blades of the propeller. Variations in rake are corrected by gradually increasing the rake in a positive direction from the hub to the tips of the blades.

While changes in the pitch and rake of blades will be effective in improving the operation of almost any blade formed from a plastic material during operation of the propeller at varying speeds, the compensations provided for in my propeller will have their greatest effectiveness when the propeller blades are relatively stiff for a plastic material. Thus, the modulus of elasticity of the plastic resin from which the propeller is formed preferably should be in the general range of about 230,000 to 410,000 p.s.i., more preferably, 310,000 to 330,000 p.s.i. Naturally, the higher the modulus of elasticity of the plastic used, the less will be the flexure of the plastic blades under the stress of operation and, consequently, the less will be the need to change the pitch and rake of the blades in their static form.

These and other objects, features and advantages of the present invention will be more apparent when taken in connection with the illustration of a preferred embodiment of my invention in the accompanying drawing, which forms a part hereof, and in which:

FIG. 1 is a front elevational view of an embodiment of my propeller, and

FIG. 2 is a sectional view along the line 22 of FIG. 1.

Referring now to the drawing, and in particular to FIG. 1 thereof, a propeller generally referred toby the numeral 10 is shown. Propeller 10 is provided with three radial blades 11, 12 and 13, formed integral with a hub 14. Propeller 10 is provided with a metallic bushing 15 fitted within the hub 14 and stationary with respect thereto. Bushing or sleeve 15 has a central aperture 16, which is shaped to slip on to and be firmly held on the drive shaft of a motor (not shown).

The pitch of each blade of propeller 10 declines in the direction of the tip of the blade. As illustrated with blade 11, which is 5% inches in length as measured along its centerline 17 and is part of a propeller designed for use with small outboard motors, the pitch decreases gradually from 12 inches near the base of the blade where it meets and is joined to hub 14 to 11 inches near the tip of the blade.

There is no absolute rule for determining the decrease in pitch of a blade in the direction of the blade tip. The percentage of decrease in pitch will vary with the strength of the plastic material from which the blade is formed, the stiffer the material, the less change in pitch required. Such change will also depend upon the number of revolutions per minute of the propeller at full speed, since maxi- 3 mum eflort should be made to compensate for variations in pitch at this r.p.m.

The view of the propeller shown in FIG. 2 is taken along the line 2-2 of FIG. 1, which includes centerline 17 of blade 11. In FIG. 2 the positive rake of the blade 11 is more apparent. With the centerline l7 shifted from its normal position to a position further aft on the boat to which the propeller is to be attached, the rake is referred to as positive. As will be seen with reference to blade 11, the centerline 17 is shifted with respect to phantom vertical centerline 18 by /16 inch at a point about halfway between the hub and the tip of the blade. Thereafter the change in rake is more severe, the rake increasing more rapidly until it reaches a maximum of inch at the tip of blade 11. All such deviation in rake is from the conventional propeller with blades mounted generally perpendicular to the hub.

The material from which the hub and blades of the propeller 10 are formed is a resin of the polycarbonate type marketed under the trademark, Lexan. Lexan is a tough thermoplastic resin having high impact strength and dimensional stability. Its modulus of elasticity is about 320,000 p.s.i. It is substantially non-water absorbent.

The gradually increasing positive rake of a propeller according to the present invention and the decrease in the pitch of the blades in a direction toward the tip of the blades combine to compensate for deformation of the blades during operation. Particularly when a plastic material of strength and resistance to deformation is utilized, the combination results in an efliciently operating, light-weight blade with all the advantages of plastic materials over metals but Without the lack of versatility and inefliciency that have marked prior attempts to operate successfully boats with plastic propellers.

It Will be apparent to those skilled in this art that some modifications and alterations can be made in the propeller illustrated and described hereinbefore without departing from the scope of the present invention. All such obvious modifications and alterations are deemed to fall within the purview of my invention, which is to be limited only by the scope of the following, appended claims.

What is claimed is:

1. A flexible boat propeller adapted for operation submerged in water, said propeller having a hub and at least one blade connected to said hub along the periphery thereof, said blade being formed from a substantially nonwater absorbent, relatively stifl" but flexible synthetic plastic material and in static condition having a pitch that decreases in a direction away from said hub, said blade extending from said hub with a positive rake.

2. A flexible boat propeller as claimed in claim 1, said plastic material having a modulus of elasticity of about 230,000 to 410,000 p.s.i.

3. A flexible boat propeller as claimed in claim 2, said plastic material having a modulus of elasticity of about 310,000 to 330,000 p.s.i.

4. A flexible boat propeller as claimed in claim 1, in which said plastic material is a polycarbonate resin having a modulus of elasticity of about 320,000 p.s.i.

5. A flexible boat propeller adapted for operation submerged in water, said propeller having a hub and at least one blade connected to said hub along the periphery thereof, said blade being formed from a substantially nonwater absorbent, relatively stiff but flexible synthetic plastic material having a modulus of elasticity of about 230,- 000 to 410,000 p.s.i., said blade in static condition having a pitch that decreases in a direction away from said hub, said blade extending from said hub with a gradually increasing positive rake.

References Cited by the Examiner UNITED STATES PATENTS 2,047,847 7/1936 Ambjornson 159 2,570,862 10/1951 Rosenkrans et al. 103-115 FOREIGN PATENTS 1,187,872 3/1959 France.

OTHER REFERENCES Modern Plastics Encyclopedia, 1963 issue, vol. 40, No. 1A, September 1962, pages 253-258.

MARK NEWMAN, Primary Examiner.

EDGAR W. GEOGHEGAN, JULIUS E. WEST,

Examiners. 

1. A FLEXIBLE BOAT PROPELLER ADAPTED FOR OPERATION SUBMERGED IN WATER, SAID PROPELLER HAVING A HUB AND AT LEAST ONE BLADE CONNECTED TO SAID HUB ALONG THE PERIPHERY THEREOF, SAID BLADE BEING FORMED FROM A SUBSTANTIALLY NONWATER ABSORBENT, RELATIVELY STIFF BUT FLEXIBLE SYNTHETIC PLASTIC MATERIAL AND IN STATIC CONDITION HAVING A PITCH THAT DECREASES IN A DIRECTION AWAY FROM SAID HUB, SAID BLADE EXTENDING FROM SAID HUB WITH A POSITIVE RAKE. 